1. Field of the Invention
This invention relates generally to a system and method for determining whether a shut-off valve is operating properly and, more particularly, to a system and method for determining whether a shut-off valve that directs a cooling fluid flowing in a fuel cell system is operating properly by determining whether a pump current for a pump that pumps the cooling fluid indicates that the valve is closed.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work can act to operate a vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more individual cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include liquid water and/or water vapor as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
It is necessary that a fuel cell stack operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. A typical stack operating temperature for automotive applications is about 80° C. The stack temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperatures above the optimum temperature may damage fuel cell components and reduce the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduces the stack performance. Therefore, fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack to maintain a thermal equilibrium.
A typical thermal sub-system for an automotive fuel cell system includes a radiator, a fan and a pump. The pump pumps a cooling fluid, such as water and glycol mixture, through cooling fluid channels within the fuel cell stack where the cooling fluid collects the stack waste heat. The cooling fluid is directed through a pipe or hose from the stack to the radiator where it is cooled by ambient air either forced through the radiator from movement of the vehicle or by operation of the fan. Because of the high demand of radiator airflow to reject a large amount of waste heat to provide a relatively low operating temperature, the fan is usually powerful and the radiator is relatively large. The physical size of the radiator and the power of the fan have to be higher compared to those of an internal combustion engine of similar power rating because of the lower operating temperature of the fuel cell system and the fact that only a comparably small amount of heat is rejected through the cathode exhaust in the fuel cell system.
The cooling fluid that is pumped through the fuel cell stack is generally also used to provide cabin heating for the passenger compartment of the vehicle. In order to provide such heating, an auxiliary loop off of the main coolant loop is provided that directs the cooling fluid to a cabin heater core that uses the heat from the cooling fluid to distribute heated air to the vehicle cabin. The cabin heater core is provided within a climate control module within the cabin of the vehicle. An electrical heater is typically provided in the auxiliary loop to raise the temperature of the cooling fluid to a temperature suitable for providing cabin heating. The heater core operates as a heat exchanger that receives the heated cooling fluid and causes air flowing therethrough to be heated. A shut-off valve is provided in the auxiliary loop that can either be opened or closed depending on whether cabin heating is desired. When the valve is open, the cooling fluid is provided to the cabin heater core to provide the heating and when the valve is closed, the cooling fluid is not available to the cabin heater core.
The shut-off valve is typically an inexpensive valve that may be susceptible to failure. If the valve is stuck in a closed position and a command is given to open the valve, the electrical heater or other components, may be damaged because the cooling fluid is not available to remove the heat. Because the shut-off valve is typically inexpensive, there is no feedback provided from the valve indicating whether it is actually opened or closed in response to a command.
Sensors and switches can be provided in and around the cabin electrical heater to detect the temperature, and switch off the electrical heater in the event that the temperature is too high, possibly because the valve has failed. However, these components and devices add cost and weight to the system, where it is desirable to eliminate the components. Additionally, the wiring harnesses and connectors that are required to be coupled to the sensors also add cost and complexity to the system. Typically, the sensors will need to be coupled directly to the electrical heater to determine whether overheating has occurred.